The present invention relates to video keyers, and more particularly to a self keyer that uses background video to fill gaps in a composited video scene.
Keying is a video process where two or more video signals are composited together into a single scene, such as a weatherman placed in front of a weather map. Currently there are two distinct algorithms for compositing two scenes into one scene: (i) a matte or additive key; or (ii) a multiplicative key.
For an additive key a fill video signal that is already shaped, i.e., the fill video signal is processed so that it appears against a black background, and a key signal that describes where the fill video signal occupies the scene are required. The additive key follows the equation: EQU Composite=Shaped Fill+Background*(1-Key)
The fill video is simply added to a hole that is cut in the background video by the key signal. A special case of a source for an additive keyer is a chroma key circuit. U.S. Pat. No. 4,409,611 issued Oct. 11, 1983 to Petro Vlahos entitled "Encoded Signal Color Image Compositing" is an example of additive keying as applied to chroma keys.
For a multiplicative key the fill video needs to be shaped or reshaped to define the portion of the fill video that is to become part of the composite. The multiplicative key follows the equation: EQU Composite=Fill*Key+Background*(1-Key)
A special type of multiplicative key is a self key where the key signal is derived from the fill signal itself. U.S. Pat. No. 4,920,415, issued Apr. 24, 1990 to Daniel J. Chaplin entitled "Self Keyer", shows an example of multiplicative self keying in FIG. 1.
The difference between the two keying methods is that the multiplicative key shapes or reshapes the fill video while the additive key simply adds the fill video to a hole cut in the background by the key signal. Although additive keys appear to be more correct since the edge of the fill is not distorted, there are many instances where self keys are used since they may give a better result visually by providing a softer appearance with less noise.
The problem with self keys is that reshaping the fill video distorts the edge of the fill video so that when it is added to the hole in the background video, the two scenes do not add up--there is a gap which shows as a black line around the fill part of the composited scene. To combat this problem traditionally the derivation of the key signal is made with a variable gain comparator which is set for high gain with a high clip value. This narrows the gap between the background and fill video edges, but is unpleasant in appearance because the edges of the resultant key are harsh and any edge noise is noticeable.
The above mentioned U.S. Pat. No. 4,920,415, to reduce the appearance of the gap, fills the gap with a variable luminance level. By varying the luminance level a balance is made between the fill and background that subjectively reduces the visual impact. Unfortunately the gray level often is only a compromise, resulting in some edges too light and others too dark, with glassware taking on a gray lackluster appearance.
Another method of reducing the appearance of the gap is disclosed in U.S. Pat. No. 5,146,333 issued Sep. 8, 1992 to Daniel J. Chaplin entitled "Self Keyer with Pre-Emphasized Fill Video". In this technique the fill signal is pre-emphasized at the edges so that the error in the self key that produces the gap is reduced.
What is desired is a self keyer that improves the appearance of composited scenes.